Much of today's modern Ethernet infrastructure is based on twisted pair copper cables that meet certain specifications. One common “category” of Ethernet cable is identified as CAT5e, which is rated for data rates up to 1 Gbps. Recently, however, proposals have been made to use the existing Ethernet infrastructure in the enterprise environment for data rates above 1 Gbps and up to 5 Gbps and beyond. Using cabling such as CAT5e and CAT6 at higher rates poses challenges such as alien crosstalk. Such cabling is typically designed for frequency ranges up to 100 MHz, yet increased data rates for new NBASE-T proposals often involve frequency ranges as high as 200 MHz and higher.
Certain NBASE-T proposals enable 5 Gbps over four twisted pairs in a bi-directional fashion, such that each pair transfers 1.25 Gbps at 400 MS/s in each direction. The signal Nyquist frequency is thus 200 MHz. A 5 Gbps NBASE-T transceiver physical layer circuit (PHY) can often deliver a reach of at least 100 m over CAT5e/CAT6 cables, where there are no aggressors. However, this reach may be limited to approximately 50 m over CAT5e and approximately 80 m over CAT6 in a “6-around-1” cabling configuration. Such a configuration arranges six cables (each having four twisted pair cables) in a radial relationship surrounding a centrally disposed cable (also having four twisted pair cables). Thus, the problem is clearly the level of alien crosstalk from adjacent cables that can significantly affect the link signal-to-noise ratio (SNR), especially when the link is long and signal attenuation is high.
In any communication link to achieve a target BER (bit error rate) at a given data rate, the signal SNR should be above a certain threshold and therefore the level of noise and interference should be contained. There may also be significant crosstalk from the other unshielded twisted pair (UTP) wires within the same cable; however, the information on the signals transmitted on each of these UTPs are generally known in the PHY and can be cancelled by linear filters.